Potential-pH diagrams at elevatedtemperature for the system Fe-H2O

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Abstract

Potential-pH diagrams at temperatures of 60, 100, 150 and 200°C are presented for the Fe-H2O system. These have been constructed in the absence of some high-temperature thermodynamic data using the extrapolation method of Criss and Cobble combined with empirical relations for the 25°C entropy of ions. The salient change in the diagrams with increasing temperature is an expanded region of corrosion in high pH media resulting from the increasing stability of the ion HFeO2. This change is discussed in relation to (1) cathodic protection, which may become more difficult at higher temperatures and pH, and (2) caustic cracking of steel, which seems to occur where HFeO2 is stable.

Résumé

On présente des diagrammes potentiel-pH du système Fe-H2O pour les températures de 60, 100, 150 et 200°C. En l'absence de certaines données thermodynamiques à haute tempature, ils ont été établis à l'aide de la méthode d'extrapolation de Criss et Cobbe combinèe à des relations empiriques de l'entropie à 25°C des ions. Le fait saillant y est l'expansion, en fonction de la température, dy domaine de corrosion en solution très alcaline due à la stabilité accrue de l'ion HFeO2. On examine les conséquences de ceci quant à la protection cathodique, qui pourrait devenir plus difficile à températures élevées, et à la fissuration caustique de l'acier, qui semble se produire là ou HFeO2 est stable.

Zusammenfassung

Es werden Potential-pH Diagramme für das Fe-H2O System für Temperaturenvon60, 100, 150 und 200°C gezeigt. Diese wurden ohne irgendwelche thermodynamische Hoch temperaturdaten angefertigt, indem die Extrapolationsmethode von Criss und Cobble im Verein mit erfahrungsgemässen Beziehungen für die 25°C Ionenentropy benutzt wurden. Der hervorstechende Wechsel in den Diagrammen mit wachsender Temperatur ist ein erweiterter Korrosionsbereich in Medias mit hohem pH, was durch die vergrösserte Stabilität des HFeOi Ion bedingt ist. Dieser Wechsel wird besprochen in Bezug auf: (1) kathodischen Schutz, welcher bei höherer Temperatur und pH schwieriger werden kann, und (2) interkristalline Rissbildung in Stahl, die anscheinend dort vorkommt, wo HFeO, beständig ist.

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